Shaft movement differential pressure measuring apparatus embodying capacitive transducers

ABSTRACT

Measuring apparatus including a capacitive transducer which comprises members between which a first capacitance increases and a second capacitance simultaneously decreases in accordance with the magnitude of the physical quantity being measured. The first and second capacitances are connected as two arms of a four-arm bridge having as each of the two other arms a charge amplifier. Means are provided for applying an AC carrier waveform to the junction of the first and second capacitances and for demodulating the differential output of the two charge amplifiers to give an output representative of the magnitude of the physical quantity.

I United States Patent 1191 1111 3, Walton June 26, 1973 [54] SHAFTMOVEMENT/DIFFERENTIAL 3,318,153 /1967 Lode 324/61 R X PRESSURE MEASURINGAPPARATUS 3,522,528 8/1970 Towner 324/61 R EMBODYING CAPACITIVETRANSDUCERS FOREIGN PATENTS ()R APPLICATIONS [75] Inventor: HymanWalton, Seascale, England 1,091,346 /1960 Germany 324/61 R 73 Assi nee:Unit d K'n d m At m' E 1 g Anthem"): i z g h Primary Examiner-Stanley T.K rawczewicz Attorney-Larson, Taylor and Hmds (22] Filed: Aug. 25, 1971[2]] App]. No.: 174,874 [57] ABSTRACT Related A fi fi Data Measuringapparatus including a capacitive transducer [63] Continuation-impart ofSer. No. 135,224, April 19, cfmprlses members between 9 a F 97L ltanceincreases and a second capacitance s1multaneously decreases inaccordance with the magnitude of [52] CL 324/61 R 73/398 C, 324/DIG lthe physical quantity being measured. The first and second capacltancesare connected as two arms of a four- [51] Int. Cl GOlr 27/26 [58] Fieldof Search....; 324/61 R, DIG. 1; arm bndge Pavmg as each 0f '9 i 73/398C charge amphfier. Means are prov1ded for applying an AC carrierwaveform to the junction of the first and [56] References Cited secondcztapatcitingles tand far demodullafting the differen 1a ou pu o e W0 0arge amp 1 1ers o g1ve an UNITED STATES PATENTS output representative ofthe magnitude of the physical 3,611,126 10/1971 Lucka 324/61 R quantity3,519,923 7/l970 Martin 2,666,896 1/1954 Harris 324/61 R 6 Claims, 7Drawing Figures 50 J/ g M 10w P485 1 a V F/[TER 01C 12/ 5/7 fie P4?/0.000 Sl/VE WAVE TEMPERATURE P t! 240 OSC/LLATOR COMPL'NSAT/NG 0 KW?20/042 201/pp MULT/PL/ER my P/- 29 /0 H0 ourpzir MON/TOR P5 D/FFERENTMLAMPZ/F/ER 5KHZ KHZ PHASE FEE/(HZ 0 "ma/7 Low P485 Sf/VS/T/Vf 4011 ms; d

F/[ TER FILTER DMODULA TOR F11 TER 1701 WIRI/IBZE GA/N SHAFTMOVEMENT/DIFFERENTIAL PRESSURE MEASURING APPARATUS EMBODYING CAPACITIVETRANSDUCERS This is a continuation in part of my Application Ser. No.135,224 filed Apr. 19, 1971.

BACKGROUND OF THE INVENTION This invention relates to measuringapparatus embodying capacitive transducers. It can be used over a widetemperature range but has one important application in making remotemeasurements in environments at above 400C.

By a capacitive transducer is meant a transducer in which the physicalquantity to be measured, eg displacement, acceleration or pressure,varies the electrical capacitance between members in the transducer.Such changes in capacitance can be measured electrically, eg in a bridgecircuit, as representative of changes in the physical quantity.

SUMMARY OF THE INVENTION The invention provides measuring apparatusincluding a capacitive transducer which comprises members between whicha first capacitance increases and a second capacitance simultaneouslydecreases in accordance with the magnitude of the physical quantitybeing measured, said first and second capacitances being connected astwo arms of 'a four-arm bridge having as each of the two other arms acharge amplifier, means for applying an AC carrier waveform to thejunction of said first and second capacitances, and means fordemodulating the differential output of said two charge amplifiers togive an output representative of the magnitude of said physicalquantity.

DESCRIPTION OF THE DRAWINGS FIGS. 1a and 1b are diagrams illustratingthe invention.

FIGS. 2a and 2b are a diagram and graph respectively indicating theperformance of the invention.

FIG. 3 is a circuit diagram.

FIG. 4 is a cross-sectional view of a capacitive transducer formeasuring pressure according to the invention.

FIG. 5 is a section on the line V-V of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1a an object which maybe a shaft is shown in a rigid body 11 which may be a housing for theshaft. The body 1 1 has secured to it two diametrically opposed plates121 and 122 havinga common conductor in a mineral insulated cable 12.The body also has secured to it diametrically opposed acceptor plates131 and 132 to which there is connected individually two conductors in amineral insulated cable 13. Earth defining plates 14 and 15 existbetween plates 121/13] and 122/132 respectively. The body 11 is earthed,as are the sheaths of the cables 12 and 13. Terminals A, B and C aremarked (See also FIG. 3). Plates 121, 131 form one pair having a leakagecapacitance between them and plates 122 and 132 form the other suchpair. The value of the leakage capacitance depends on the width of thevariable gap between object 10 and plates 14 or 15.

In FIG. lb the same reference numerals are used as in FIG. 1a butthe'plates 121, I22, 131 and 132 are now carried by the object 10 whichmay be a removable nuclear fuel element instead of by the rigid body 11which may be the permanent structure of a nuclear reactor. The body 11is earthed whereas, in FIG. 1a the object 10 was earthed.

In FIGS. 20 and 2b, FIG. 2a being in effect an enlargement of a part ofFIG. 1b, the performance of the invention is considered especially inrelation to the position occupied by earth defining plate 14/15. In FIG.2a the degree of projection of the earth plate 14/ l 5 beyond the plates122, 132 is indicated by the dimension d. The distance between the edgeof plate 14/15 and the earthed body 11 is indicated by the dimension D.In FIG. 2b there are three curves a, v and c which show the performanceof the invention of the various d/D ratios indicated.

In FIG. 3 the cable 12 is connected to a 20 KHz oscillator 20, whichprovides a carrier waveform and also has an output connector 21 to aphase-sensitive demodulator 22. The capacitances 131, 132 can beconsidered as two arms of a bridge, the other two arms being chargeamplifiers 231 and 232 connected to the capacitances 131, 132 by thecable 13. Capacitors 240, 241, 242 provide a calibration and balancecircuit, a switch 243 acting as a calibration switch. (A chargeamplifier is a high-gain D.C. amplifier with capacitative feedback whichproduces an output voltage directly proportional to the change of chargeat its input terminals) Capacitor 241 is adjustable for zero setting.The output of the charge amplifiers is taken to a variable gaindifferential amplifier 25 and thence to a band-pass filter of 5 KHz bandwidth centered on the oscillator frequency of 20 KHz. The output fromthe demodulator 22 is taken to another variable gain differentialamplifier 27 and thence through a low-pass filter 28 which is set topass signals up to 2.5 KHz. The output is monitored on a meter 29 whichcan be calibrated in units of displacement of the object 10.Alternatively the output could be recorded on magnetic tape, forsubsequent analysis, associated with the output terminal 0.

The cables 12, 13 may be long, one of the features of the invention, andthis length is indicated by the break lines in the cables.

Preferably cable 12 is a thermocouple cable which monitors thetemperature at the transducer represented by ABC in addition to servingits prime function of leading the 20 KHz drive signal to the transducer.The cable 12 is connected to a differential amplifier 30 the output ofwhich is directly proportional to the transducer temperature. Thisoutput is passed to a low pas (5 Hz) filter 31 before being fed to atemperature compensating multiplier 32 also connected with thedisplacement output from filter 28. From terminal P of the multiplier 32a temperature compensated signal appears.

In operation, the oscillator 20 feeds plates 121 and 122 with a 20 KHzcarrier signal. These plates have leakage capacitances to the plates 131and 132 respectively. These leakage capacitances are varied by anydisplacement of the object 10 because such displacement varies thewidths of the gaps at the edges of plates 14 and 15. Thus the carrierwaveforms fed to the conductors of cable 13 via these capacitances aremodulated by the displacement of object 10. With diametrical dispositionof plates 131 and 132 the overall mechanical and electrical symmetry,the modulated signals in the two conductors are in anti-phase. Thesesignals receive amplification in amplifiers 231 and 232 and from these adifferential signal is obtained from amplifier 25. The output from thefilter 26 is demodulated and identified as positive or negative in therespective outputs of demodulator 22. The amplifier 27 provides a D.C.output.

A typical size for the plates 121:122zl31 and 132 is 6.25 sq. ems togive a capacity of 0.05 pica-farade. The capacitor 242 is 10 pF andcapacitor 241 ranges over 20 pF. Capacitor 240 is 10,000 pF.

For the operation of the temperature compensating circuit, thetemperature signal derived from the filter 31 controls, in themultiplier 32, the amplitude of the displacement signal obtained fromfilter 28 by varying a multiplier coefficient. Thus, as the temperaturerises, the coefficient will be reduced and when the temperature fallsthe coefficient will be increased. Temperature compensation is usefulwhen the transducer is used as an accelerometer or berometer as it cancompensate for changes with temperature of dynamic Youngs Modulus of thematerials in the transducer.

In FIGS. 4 and 5 the transducer is of a kind suitable for measuringdifferential pressure. It comprises a pressure casing 40 in four partswhich can, for high temperature and pressure use, be edge weldedtogether. Within the casing 40 there are two diaphragms 41, 42 dividingthe easing into three chambers 43, 44 and 45. Differential pressurepipes 46, 47 are connected respectively to chamber 43 and 45. The centerchamber is vented either to atmosphere, or to any convenient pressurenear to the pressures in pipes 46, 47. The flexibility of the diaphragmsis confined principally to their edge regions, the center regions beingrobust and equipped with push rods 48, 49 which bear against acantilever beam 50. The beam carries two linked capacity plates 121, 122insulated from the beam. Plates 131, 132 are placed 0.020 inches awayfrom plates 121 and 122 and are physically connected to the transducerbody but insulated from it. A twin mineral insulated coble 13 connectsthe plates 131, 132 to charge amplifiers (as in FIG. 3) and athermocouple cable 12 supplies the oscillator carrier signal to theplates 121, 122.

Operation follows from that known for differential pressure transducers,namely, the beam 50 deflects according to the differential pressures inchambers 43 and 45 due to action by the push rods moving with deflectionof diaphragms 41, 42. The movement of beam 50 increases the capacitancebetween plates 122 and 132 and decreases that between plates 121 and131, or vice versa, depending on the direction of deflection of thebeam. These two capacitances are connected in a four-arm bridge as inFIG. 3, and the beam deflection determined accordingly as a measure ofthe differential pressure. It will be seen that, unlike FIGS. 1 and 2,the transducer of FIGS. 4 and 5 does not use changes in leakagecapacitance.

In an alternative, non-preferred, form of differential pressuretransducer, changes of leakage capacitance between pairs of relativelyfixed members are measured, as in FIGS. 1 and 2. In this alter-nativetransducer the beam 50 is omitted and the diaphragms 41, 42 replaced bybellows of longer travel whose free" ends control leakage capacitancesas they move, in the same manner as does the movement of object relativeto the pairs of plates in FIG. 1a. However, longtravel bellows produce aless robust construction and introduce a dependency on bellowscharacteristics. By contrast, the diaphragms 41, 42 in FIGS. 3 and 4 areprincipally pressure boundaries with a very small deflection and have alow spring rate compared with beam 50.

Apparatus according to the present invention, comprising a transducerwhich includes a cantilever beam as in FIGS. 4 and 5, can be used as anaccelerometer. The diaphragms and push rods are omitted in this case andthe free end of the beam made suitably massive.

The invention allows long lengths (typically 30 meters) of cable 12/13to be used, as noise unavoidably generated in these cables and picked upexternally is cancelled out, and the lead/earth capacitance of thesecables which may be as high as 4,000 pF is of no relevance. The filter26 greatly assists in the elimination of environmental noise.

With larger plates 121, 122, 131, 132 it would be possible to detectmovements of humans passing between the plates and, in this respect, theinvention could provide a burglar alarm. As the invention is based oncapacitance measurement it could also detect dielectric displacementssuch as would occur when an air dielectric of one humidity was displacedby a dielectric of changed humidity.

I claim:

1. Measuring apparatus comprising a capacitive transducer comprisingmembers providing a first capacitor and a second capacitor, thecapacitance of at least one of said capacitors being variable inaccordance with the magnitude of a physical quantity being measured; afour arm bridge; means for connecting said capacitor as two arms of saidfour arm bridge; two charge amplifiers; means for connecting said chargeamplifiers so that said amplifiers and the inputs thereto form the othertwo arms of said four arm bridge; means for applying an AC. carrierwaveform across the junction of said capacitors and the junction of theinputs of said amplifiers; means for deriving a differential outputsignal from the outputs of said charge amplifiers; and means fordemodulating said differential output signal to produce an outputrepresentative of the magnitude of said physical quantity.

2. Apparatus as claimed in claim 1 wherein said transducer whichcomprises the capacitor arms of said bridge is remotely located relativeto the charge amplifiers and said means for applying said carrierwaveform, said apparatus further comprising groundsheathed cables forestablishing connections between said transducer on the one hand andsaid charge amplifiers and said means for applying said carrier waveformon the other hand.

3. Apparatus as claimed in claim 1 wherein said means for applying saidcarrier waveform includes a thermocouple cable, said apparatus furthercomprising a thermocouple for registering the temperature at saidtransducer and generating an e.m.f. in accordance therewith, atemperature compensating multiplier, connected to accept said outputrepresentative of the magnitude of said physical quantity, for modifyingsaid output to compensate for the effects of temperature changes at thetransducer, and means for utilizing the e.m.f. of said thermocouple tocontrol said temperature compensating multiplier.

4. Apparatus as claimed in claim 1 wherein said members include a firstmember located between two further members and arranged to be displacedrelative to said two further members.

5. Apparatus as claimed in claim 4, wherein the first said membercomprises a deflectable cantilever beam 6. Apparatus as claimed in claim5, for measuring a pressure differential, wherein the transducercomprises a diaphragm means linked to each side of said cantileand saidtwo further members are rigidly connected to 5 Vet beam by P rod mfiansfor deflecting Said beam-

1. Measuring apparatus comprising a capacitive transducer comprisingmembers providing a first capacitor and a second capacitor, thecapacitance of at least one of said capacitors being variable inaccordance with the magnitude of a physical quantity being measured; afour arm bridge; means for connecting said capacitor as two arms of saidfour arm bridge; two charge amplifiers; means for connecting said chargeamplifiers so that said amplifiers and the inputs thereto form the othertwo arms of said four arm bridge; means for applying an A.C. carrierwaveform across the junction of said capacitors and the junction of theinputs of said amplifiers; means for deriving a differential outputsignal from the outputs of said charge amplifiers; and means fordemodulating said differential output signal to produce an outputrepresentative of the magnitude of said physical quantity.
 2. Apparatusas claimed in claim 1 wherein said transducer which comprises thecapacitor arms of said bridge is remotely located relative to the chargeamplifiers and said means for applying said carrier waveform, saidapparatus further comprising ground-sheathed cables for establishingconnections between said transducer on the one hand and said chargeamplifiers and said means for applying said carrier waveform on theother hand.
 3. Apparatus as claimed in claim 1 wherein said means forapplying said carrier waveform includes a thermocouple cable, saidapparatus further comprising a thermocouple for registering thetemperature at said transducer and generating an e.m.f. in accordancetherewith, a temperature compensating multiplier, connected to acceptsaid output representative of the magnitude of said physical quantity,for modifying said output to compensate for the effects of temperaturechanges at the transducer, and means for utilizing the e.m.f. of saidthermocouple to control said temperature compensating multiplier. 4.Apparatus as claimed in claim 1 wherein said members include a firstmember located between two further members and arranged to be displacedrelative to said two further members.
 5. Apparatus as claimed in claim4, wherein the first said member comprises a deflectable cantilever beamand said two further members are rigidly connected to the casing of thetransducer.
 6. Apparatus as claimed in claim 5, for measuring a pressuredifferential, wherein the transducer comprises a diaphragm means linkedto each side of said cantilever beam by push rod means for deflectingsaid beam.